Avalanche photodiode

ABSTRACT

AN AVALANCHE PHOTODIODE, PARTICULARLY USEFUL FOR DETECTION OF INFRARED ENERGY IN THE WAVELENGTH REGION FROM 1 TO 2 MICRONS, INCLUDES AN ACTIVE EPITAXIAL LAYER OF A QUATERNARY IIIV ALLOY SUCH AS N-IN-1-YGAYP1-XASX. THE ACTIVE LAYER INTERFACES WITH A SECOND LATTICE MATCHED EPITAXIAL LAYER OF P-TYPE JUNCTION AL TO DEFINE A LATTICE MATCHED P-N JUNCTION THEREBETWEEN WHICH IS REVERSED BIASED, IN USE. THE ACTIVE LAYER HAS A DIRECT BANDGAP ENERGY LESS THAN THE PHOTON ENERGY OF THE INFRARED ENERGY TO BE DECTED FOR ABSORPTION OF THE PHOTONS TO BE DETECTED TO GENERATED ELECTRON-HOLE PAIRS. THE ACTIVE ALYER IS MADE OF A MATERIAL HAVING AN ENERGY DIFFERENCE BETWEEN THE LOWEST CONDUCTION BAND MINIMUM BAND THE NEXT HIGHER CONDUCTION BAND MINIMA EITHER X1 OR L1 WHICH IS GREATER THAN 1.1 TIMER THE DIRECT BANDGAP ENERGY OF THE ACTIVE LAYER, WHEREBY IMPROVED SIGNAL TO NOISE RATIO IS OBTAINED.

United States Patent 1 James [11]. 3,821,777 June 28, 1974 AVALANCHEPHOTODIODE Kressel 148/171 OTHER PUBLICATIONS "Shih et a1., 1.B.M. Tech.Discl. Bull. Vol. 11, N0. 12,

Primary Examiner-Martin H. Edlow Attorney, Agent, or FirmStanley Z.Cole; Paul l-lentzel [57] ABSTRACT An avalanche photodiode, particularlyuseful for detection of infrared energy in the wavelength region from 1to 2 microns, includes an active epitaxial layer of a quaternary lll-Valloy such as n-ln-, ,,Ga,,P, ,As,. The active layer interfaces with asecond lattice matched epitaxial layer of p-type material to define alattice matched p-n junction therebetween which is reverse biased, inuse. The active layer has a direct bandgap energy less than the-photonenergy of the infrared energy to be detected for absorption of thephotons to. be detected to generate electron-hole pairs. The activelayer is made of a material having an energy difference between thelowest conduction band minimum and the next higher conduction bandminima either X or L which is greater than 1.1 times the direct bandgapenergy of the active layer, whereby improved signal to noise ratio isobtained.

1969' 10 Claims, 9 Drawing Figures I l2 We ---.|0ov- L e 9 8 Mn 5m n-lnGaAsP M Zps F'InGoAsP PIIDP 1P 2 5115.

' n v I 200JJ 2 Q S'O 0R SiO AMREF-LECHON i ACTIVE P-n JUNCTION 7COATING l4 IR PHOTON PATE'N'T'EDaunza 1914 1 777 samura I5 I L I fil 9NiSn w ATZOO H 5,15 n-InGuAsP u n )(A Zps P 'II'IGCIASP P11 P/J s InP I200115 2 F|G SiO OR sio ANMEFLECHON ACTIVE P n JUNCTION 7 COATING l4 IRPHOTON FIG.2

g 'GoSb 5 DIRECT m BANDGAP "INDIRECT 0 I 5.4 5.5 5.6 57 5.8 5.9 6.061BANDGAP LATTICE cowsmm ('A) CONDUCTION BAND PATENTEDJUNZG I974 SNiET 20T 4 INDIRECT sfsofi GoAs |42ev 65A E m l T A IL 0 S I CONSTANT ISOBANDGAP GOP 2.2ey

X INCREASING MOLEFRACTION InAs FIG.4

InP

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|58ev OFASINTHEISUBLATTICE 588v 585K 605K INDIRECT |.o InAs AvALANcimPHOTODIODE DESCRIPTION or THE PRIOR ART Heretofore, avalanche infraredphotodiodes have employed ternary III-V alloy active layers of GaAsSbgrown upon a binary III-V alloy substrate of GaAs. Such prior artavalanche photodiodes have been plagued with excessive noise from twosources. A first source of noise comes about because the averagemultiplication gain for the avalanche diode varies as a function ofposition over the p-n junction area due to junction nonuniformities.Secondly, the distribution of the actual gain for each photoelectron, ata given point in the junction, isvery wide due to the statistics of theavalanche multiplication process.

In these prior art infrared detectors, the active ternary material had alattice constant substantially different than that of the binarysubstrate, i.e. approximately 1% mismatch. This mismatch in latticeconstants caused dislocations to be introduced during the materialgrowth. These dislocations make it extremely difficult to obtain uniformjunctions. Nonuniform junctions cause nonuniform average multiplicationas a function of position over junction area, thereby constituting thefirst source of noise previously discussed.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention isthe provision of an improved avalanche photodiode sensitiveto infrared radiation in the I to 2 micron wavelength region.

In one feature of the present invention, the active layer of thephotodiode, in which infrared photons are absorbed to generateelectron-hole pairs, is made of a material having an energy differenceEr and En.

between the lowest conduction band minimum (R) and the next higherconduction band minima either X or L which is greater than I I times thedirect bandgap energy of the active layer, whereby the active layer ofthe avalanche photodiode has a favorable band structure resulting in animprovement in. the distribution of gain, that is, a smaller spreadinthe number of electrons out of the device into the external circuitfor one electron entering the avalanche region, whereby improved signalto noise ratio is obtained.

In another feature of the present invention, the active layer of theavalanche photodiode is made of a quaternary III-V alloy of ln ,,Ga P Aswhere y falls within the range of 0.6 and 0 and x falls within the rangeof 0.45 to 1, whereby the active layer has a favorable band structurefor improved signal to noise ratio.

In another feature of the present invention, the active layer andadjoining interfacing layer, defining the p-n junction, are grown upon acommon substrate such substrate having a lattice constant matched to thelatter constant of the active p-n junction forming layers, wherebyhighly uniform junctions are obtained to yield uniform average gainacross the junctions resulting in improved signal to noise ratio.

In another feature of the present invention, the substrate islnP havinga bandgap energy above the photon energy of the infrared radiation to bedetected and the active p-n junction forming layers are made of aquaternary IIl-V alloy of InGaPAs proportioned to have a bandgap energybelow the photon energy of the infrared energy to be detected, wherebythe substrate is BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematicsectional view of an avalanche photodiode incorporating features of thepresent invention,

FIG. 2 is a plot of bandgap energy in electron volts vs. latticeconstant in angstroms-showing the diagram for the quaternary III-V alloyof GalnAsP and the direct and indirect bandgap regions thereof,

FIG. 3 is a plot of energy vs. momentumvector showing the conductionband and valance bands for a typical III-V alloy,

FIG. 4 is a plot of isobandgap lines and isolattice constant linessuperimposed upon the compositional plane for the quaternary III-V alloyof ln Ga P As FIG. 5 is a plot of bandgap energy in electron volts andthe X conduction band energy minima relative to the top of the valanceband in electron volts superimposed upon the compositional plane for theInGaPAs quaternary III-V alloy,

FIG. 6 is a plot similar to that of FIG. 5 depicting the L conductionband energy minimum relative to the top of the valance band superimposedupon the compositional plane,

FIG. 7 is a plot of bandgap energy superimposed upon the compositionalplane for a quaternary III-V alloy of InGaPAs depicting the latticeconstant line for InP and the high noise and low noise region of thematerial when utilized as the active layer for avalanche diode use,

FIG. 8 is a plot of conduction band energy and valenceband energy as afunction of distance through the laye'rs of the avalanche photodiodestructure of FIG. 1, and

FIG. 9 is a plot of photo electron yield per incident photon vs. photonenergy for the photodiode of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, thereis shown an infrared sensitive Iownoise avalanche photodiode Iincorporating features of the present invention. More particularly, theavalanche diode 1 includes a single crystal substrate 2, as of 200microns thick, and made of a material transparent to the infraredradiation to be detected. This requires that the substrate material havea bandgap energy above the photon energy of the infrared photons to bedetected. For detection of photons corresponding to wavelengths in the 1to 2 micron region a particularly suitable substrate material is thebinary III-V alloy of InP which is preferably undoped to minimize freecarrier absorption.

A heavily p-doped (p-dopant concentration 3 X 'I0' /cm epitaxial layer,3, as'of five microns thick, is grown upon the substrate 2 to provide anelectrical contact region to an active layer 4 of the diode l. Aparticularly suitable material for the electrical contact layer 3, whenthe substrate is InP, is In? where layer 3 is lattice matched to layer 2and has a desirable bandgap energy (i.e. greater than the photon energyto be 3 detected) to minimize absorption of photon energy to bedetected.

Lattice matching to the substrate eliminates lattice dislocations thatcould otherwise propagate through the sandwich like structure tovproduce junction nonunifor'mities.

A uniform junction reduces the distribution of average multiplication asa function of position over the junction area, thereby reducing thefirst source of noise previously discussed. Thus, lattice matching atthe junction of Iayer'4 to the substrate will give few dislocauniformaverage gain across the junction. The active layer 4is preferablylattice matched to the substrate'2 and to the electrical contact region3 of the substrate and is made of a material having a bandgap energyslightly less than the lowest photon energy of the infrared photons tobe detected, such that the photons passing through the substrate will beabsorbed in the active layer 4 for generation of electron-hole pairs.

A particularly suitable active layer 4'is a lightly pdoped (i.e.p-dopant concentration lo /cm?) quaternary llI-V alloy of lnGaAsP havingits constituents proportioned for lattice matching to theInP substrateand with the constituents also proportioned to provide the desireddirect bandgap below the photon energy to be detected; This material isalso particularly desirable in that it has ade'sirable band structurewherein the energy difference Er; or Err between the lowest conduc tionband minimum (F and-the next higher conduction band minima, either theX, or L, minima, which is greater than 1.1 times the direct bandgapenergy of the active layer, whereby the conditions for low noiseavalanche gain are achieved, as more'fully described below, and wherebythe second source of noise is minimized. A particularly suitablecomposition for the active layer utilized in combination with an InPsubstrate is that as shown by the cross-hatchedregion 5 of line 23'ofFIG. 7, and particularly the point marked A along that line having acomposition as follows: ln Ga 41 0.s9 o.11-

The active layer .4 is the layer in which the photon absorption takesplace. It must be thick enough so that almost all of the light isabsorbed in, this region, otherwise excess noise will be generated dueto the differing multiplication gain for holes generated in theoverlying ntype region. The actual thickness, T, and doping density oflayer 4 is optimized to provide maximum demodulation frequency 1response.

A p-n junction forming heavily n-doped (i.e. n dopant concentration asuitable p-type ohmic contact electrode 11, as of an alloy of Au and Zn,is formed on the pilayer 3 and an electrical circuit consistingof asource of potential 12 as of 100 volts and a series load resistor R 13is connected in series withlthe contacts 8 and 11 for reverse IO /cmepitaxial layer 6 isformed overlaying and interfacing with the'activelayer tions which will result in higher uniform junctions and 4 biasingthe active p-n junction 7. An anti-reflection coating 14, as of SiO,, orSiO, is formed on the substrate 2 along the beam path for the photons tobe detected for minimizing the reflection of photons fromthe diode 1.The device is etched to form the conventional mesa configuration.

In operation, infrared photon energy having wavelengths in the range ofl to 2 microns pass through the anti-reflection coating 14, substrate 2,conductive region 3, and into the active layer'4 wherein they areabsorbed to produce electron-hole pairs. Theelectrons are acceleratedacross the reverse bias potential of the p-n junction and generate extraelectron-hole pairs due to ionization. The carriers 'are swept out ofthe junction and cause a current to flow in the external circuit throughthe load resistor R 13 as detected current I producing an output voltageacross terminals '15. The avalanche diode l of FIG. 1,. whichincorporates features of the present invention, provides improved signalto noise ratio and has a quantum efficiency of approximately 70 percent,as shown in FIGS. 9.

Use of lnGaAsP lattice matched to the substrate as the active layer 4has several advantages over the prior ternary III-V alloy activelayersformed on binary IlI-V substrates. This quaternary material has an extradegree of freedom, as contrasted with theternary material, in that thebandgap and lattice constant can be independently specified (within agiven range). Thus, the bandgap energy of the active layer can be chosento optimize performance at the desired wavelength and the latticeconstant of the active layer can be chosen to provide a lattice match tothe binary substrate material and to the p-n junction forming layer,thus, giving uniform junctions and uniform gain.

This can be seen by examination of FIG. 2 which plots the bandgap as afunction of lattice constant for some III-V materials including thesubject quaternary material. The binary materials, which arecommercially available in suitable form for use as substrate material,are shown as the small open circles in FIG. 2. The ternary materialssuch as InGaAs, GaAsSb, and InAsP are shown by the curved linesconnecting the two binary end points. Notice that for any of the ternarymaterials (except GaAlAs which has too high a bandgap range to be ofinterest as an infrared detector) a change in bandgap requires asignificant change in-lattice constant. For example, an avalanchephotodiode designed for 1.06 micron operation which uses GaAsSb grown ona GaAs substrate requires GaAsSb with a bandgap of about 1.1 eV(indicated as point 10 on FIG. 2). This material has a lattice constantof greater than 5.7 angstroms giving a lattice mismatch of 1 percent ormore to the GaAs substrate.

The lnGaAsP quaternary is representedin FIG. 2 by the shaded area. Aunique composition of lnGaAsP exists which gives thelattice constant andbandgap energy specified by any point within the indicated area. Forexample, in the case of a 1.l6 eV bandgap material, required as in theexample above, it is possible to choose a material-with this bandgap andwith a lattice constant exactly matched to that of an In? substrate.This is indicated by point 20 in FIG. 2. As a matter of fact, it ispossible to obtain quaternary Ill-V materials with exact latticeconstant match to InP and with bandgaps ranging from 0.75 to 1.34 eV.This set of lnGaAsP material is indicated in FIG. 2 by the heavyvertical line 21 drawn downwardly from the lnP point. This latticematching will give few dislocations, which will result in highly uniformjunctions, and in uniform average gain across the junction, therebyreducing the noise generated by the first source of noise previouslydiscussed.

The second advantage of the quarternary III-V alloy of InGaAsP as theactive layer 4 is the improvement it offers in the distribution of gain,that is, a smaller spread in the number of electrons out of the deviceinto the external circuit for one electron entering the avalancheregion. This property of the InGaAsP material results from its desirableband structure.

As was shown by McIntyre, in IEEE Transactions On Electron Devices, Vol.9, p. 703 (1972), the statistical noise fluctuation of the gain dependsupon the ratio of the hole and electron ionization coefficients. Forequal ionization coefficients the noise is proportional to M where M isthe average multiplication. For a very large ratio of ionizationcoefficients (ionization by one carrier only) the noise is proportionalto (ZM -M). Thus for typical gains of M 300 to 1,000, the noisecharacteristics are greatly improved with a large ratio of ionizationcoefficients. The ratio of ionization coefficients for a materialdepends on its band structure. With In- GaAsP we are able to choose amaterial composition having the desired band structure for low noiseoperation.

The band structure for a typical III-V material is shown in FIG. 3. Thedirect bandgap is indicated as E,,. The energies of the X, and Lconduction band minima are given relative to the top of the valence bandby E and E respectively. FIG. 4 shows E, as a function of compositionfor InGaAsP. The lattice constant is also shown in FIG. 4. FIG. 5 showsthe value of E as a function of composition, and FIG. 6 shows thevalueofThe ratio of electron-to-hole ionization coefficients' depends on therelationships among the energies of the conduction and minima. Aselectrons approach the high field region at the reverse biased p-njunction of the device, they are in a thermalized distribution at thebottom of the lowest conduction band minimum (I Upon entering the highfield region they will be accelerated to higher and higher energies inthe I, minimum until one of two things happen. If they reach the energyof the next higher lying conduction band minima (either X or L beforegaining sufficient energy to create an electron hole pair, they will betransferred by optical phonon scattering to one of those minima. This isthe basis of the Gunn effect. The electrons will then continue to beaccelerated in the upper minima until they reach a high enough energy tocreate an electron-holepair. Because the X and L minima have effectivemasses very similar to that of the valence band, the electric fieldrequired to reach an energy sufficient to cause ionization ispractically the same for holes and electrons, and materials of this typewill have nearly equal hole and electron ionization coefficients. GaAs,GaAsSb, and InGaAs with bandgaps of 0.95 eV and above all fit into thisclass of material.

The other case, the low noise case, is where the band structure is suchthat an electron can gain enough energy in the F minimum to create anelectron-hole pair without reaching the energy of the upper conductionband minima. In this case, because the effective mass in the F minimumis much lower than the valence band effective mass, a much lowerelectric field is required 9rl9tr9ns tq ea zati ne gy, tha or q qs-Thus, the ratio of electron to hole ionization coefficients is verylarge. An avalance photodiode made from a material of this type willshow much better noise performance. Another way of stating the samepreferred qaditignisth t nrs. ...,s1 stsn. Eri .wd Eu. between thelowest conduction band minimum (I,) and the next higher conduction bandminima either X, or L is enough greater than the direct bandgap energyof the active layer 4 so that a hot electron is more likely to generatean electron-hole pair than to transfer by scattering to anotherconduction band minimum.

The energy required for ionization is somewhat greater than the bandgapenergy because momentum 'and energy must be conserved in the ionizationevent.

For III-V direct bandgap materials, the required energy is about 1.1times the bandgap energy, E,,. Thus, the requirements for a material tofit into the desirable category of the low noise case are that Er 1.1 E,and EFL 1.1 E,,. As may be seen from FIGS. 4, 5 and 6, In-GaAsPsatisfies these conditions over part of its composition range. This isindicated in FIG. 7 by the broad solid line 22. Material with acomposition below and to the right of line 22 satisfies the conditionsfor low noise avalanche gain. Materials to the left and above line 22 donot. Thus, the active layer 4 of the avalanche photodiode 1 shouldpreferably be made of InGaAsP with composition falling to the right andb low line .22-

Also shown in FIG. 7 is the constant lattice line 23 for those materialswith an exact lattice method to InP. Notice that there exists a range ofmaterials, indicated by the cross-hatched pattern 5 along line 23 inFIG. 7 which satisfies both the substrate lattice matching criteria andthe band structure criteria for low noise gain. Thus, by causing thecomposition of the InGaAsP active layer material to fall within thecross-hatched region 5 of line 23 the avalanche photodiode I will havevery uniform gain across the p-n junction and the lowest possible noiseadded by avalanche gain when the diode is constructed upon an InPsubstrate. A particularly suitable material falling within thecross-hatched region 5 of line 23 is that as indicated at point A havinga composition of 1m Ga AS039 P The epitaxial layers 3, 4 and 6 aregrown, preferably by liquid phase epitaxy on the InP substrate 2. Theprocedure used for growing the epitaxial layers consists of preparing aseries of ln-Ga-As solutions with increasing amounts of Ga and thensaturating the solution with P. Following equilibrium, the melt isbrought in contact with a single crystal substrate of In? (which may bethe l l l A or B oriented face of the face of the material). Uponcontacting the melt with the single crystal substrate at a suitabletemperature, as of 650C, a controlled cooling cycle is initiated fordropping the temperature by 25 to 50 depending upon the thickness of theepitaxial layer to be grown. Cooling rates may be varied between 2.0 and0. 1 C per minute with no apparent dependence of the surface structureof the peitaxial layer on cooling rate.

The lattice constant of the epitaxial layer is measured, for example, byX-ray deffraction of Cu-Kalpha radiation. Bandgap energy in theepitaxial layer is determined by photoluminescense techniques at bothroom temperature and reduced temperature, if desired. A typical devicefor measuring the bandgap energy is to illuminate the material with amonochromatic light source, such as a 0.5 watt argon ion laser beam andto j observe the reflected light with a spectrophotometer, such asPerkin Elmer 301 spectrophotometer using a dry ice-cooled Splphotomultiplier.

A preferred method for growing the epitaxial layer is the conventionalsliding bin method in a purified hydrogen atmosphere, according to themethod of MB. Panish et al as described in an article tilted Preparationof Multi-layer LPE heterostructures with Crystalline Solid Solutions ofAl, (la, As Heterostructure Lasers appearing in the MetallurgicalTransactions of the AIME, Volume 2, pages 795-801 (March 1971 A methodfor growing lattice matched layers of In,.,, Ga, P As, on III-V alloysof In, Ga, P, and As is disclosed and claimed in copending US.application filed and assigned to the same assignee as the presentinvention.

As used herein lattice matched means that the lattice constants arematched to within 0.5 percent, and quaternary III-V alloy of InGaAsP isdefined to mean that the elements are proportioned according to theformula: In, Ga As, P

What is claimed is:

1. An avalanche photodiode which responds to incident photons byproviding photo-excited electrons which produce avalanche electrons,comprising:

a p-doped semiconductive layer for absorbing the incident photons toprovide photo-excited electrons;

an n-doped semiconductive layer epitaxially interfaced and latticematched with the p-doped layer for defining a p-n junction along theinterface therebetween;

means for establishing a reverse bias across the p-n junction; and

the p-doped semiconductive layer being made of ln Ga P As in whichvaries from about 0.6 to and x varies from about 0.45 to 1.0 and inwhich the lowest conduction band minima (r and either of the next twolowest conduction band minima (x, or L have an energy difference (Erx orErl) which is sufficiently greater than the bandgap energy of thep-doped semiconductive layer to establish a probability of greater than0.5 that the photoexcited electrons within the p-doped layer willaccelerate under the reverse bias and generate avalanche electrons,rather than transfer by phonon scattering to one of the conductionminima.

2. The photodiode of claim 1 wherein a substrate is provided forsupporting the semiconductive layers.

3. The photodiode of claim 1 wherein E and E are greater than 1.1 timesthe direct bandgap energy of the p-doped layer.

4. The photodiode of claim 2 wherein the substrate portion isepitaxially interfaced to the photodiode and is made of a III-V alloyselected from the group consisting of In, Ga, P, and As.

5. The photodiode of claim 4, wherein the interfacing portion of thesubstrate portion is InP.

6. The photodiode of claim 4 wherein the interfacing portion of thesubstrate is made of a p-type in? material for making electrical contactto the p-doped semiconductive layer, and wherein the remainder of thesubstrate along the path of the incident photons is made of an undopedin? material to minimize unwanted free carrier absorption.

7. The photodiode of claim 6 including, an antireflection coating on thesubstrate disposed in alignment with the path of incident photons toavoid unwanted reflection of incident photons.

8. The photodiode of claim 5 wherein the material of the p-dopesemiconductive layer is ln Ga 41 o.ss o.n-

9. The photodiode of claim 5 wherein y is greater than 0.40 and lessthan 0.55, and wherein x is greater than 0.8 and less than 1.0.

10. The photodiode of claim 1 wherein both of the semiconductive layersare made of quaternary Ill-V alloys of In Ga P As UNITED STATES PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3,821,777 Dated June 28,1974 Inventor-(s) Lawrence James It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Claim 1, column 7, line 37,

Change 1: t line 38, change "x to "1' I n n line 39, change E or E to Eor E Claim 3, column 8, line 10,

II II change E and E to E and E Signed and sealed this 10th day of June1975.

(SEAL) Attest C. MARSHALL DANN RUTH C. MASON Commissioner -;of PatentsArresting Officer and Trademarks

